The invention relates to the electrodeposition of cadmium and, particularly to a plating bath for plating bright level cadmium deposits from aqueous acid cadmium baths. More particularly, the invention relates to the incorporation into the acid cadmium baths of at least one thiourea compound. The invention also relates to methods for electrodepositing level and bright cadmium deposits from such baths.
A variety of plating baths have been developed and employed for electroplating cadmium onto metallic substrates. These baths typically utilize sulfates and cyanides as the primary electrolytes. The cyanide baths have proven effective and generally satisfactory despite certain objectionable features such as high toxicity, low current efficiency and hydrogen embrittlement of certain steels. The sulfate baths which have been suggested overcome many of the objectionable features of the cyanide baths. However, some of the sulfate-based baths contain such components as ammonium ions and chelating agents. Because of the ability of these agents to complex with heavy metal ions, there is a significant increase in the difficulty of eliminating heavy metals from spent baths. Sulfate-based baths which do not utilize ammonium ions or chelating agents have more recently been suggested in the prior art. For example, in U.S. Pat. No. 3,998,707, an aqueous acidic cadmium electrolytic bath composition is described which comprises cadmium ions, free acid, and a particular surfactant combination which comprises a cationic polyoxyalkylated amine and an anionic surfactant. Preferably the cadmium plating bath also contains at least one brightener. Examples of brighteners include aryl aldehydes such as anisic aldehyde, ring halogenated aryl aldehydes such as orthochlorobenzaldehyde, heterocyclic aldehydes such as thiophenealdehyde, aryl olefinic-conjugated ketones such as benzylidine acetone, and heterocyclic carboxylic acids such as nicotinic acid. The combination of orthochlorobenzaldehyde and benzoyl pyridine is shown in Example 3. Similarly, U.S. Pat. No. 4,045,305 describes cadmium plating baths which contain cadmium ions, free acid, a surfactant combination comprising a condensed naphthalene sulfonate compound and a non-ionic polyoxyalkylated surfactant. Preferably this bath also contains a brightener of the type described in U.S. Pat. No. 3,998,707.